UV visible spectrum of quercetin, with lambda max at 369 nm
Quercetin is a plantflavonol from theflavonoid group ofpolyphenols. It is found in many fruits, vegetables, leaves, seeds, and grains; capers, red onions, andkale are common foods containing appreciable amounts of it.[2][3] It has abitter flavor and is used as an ingredient indietary supplements, beverages, and foods.
Quercetin is aflavonoid widely distributed in nature.[2] The name has been used since 1857, and is derived fromquercetum (oak forest), after the oak genusQuercus.[4][5] It is a naturally occurringpolar auxin transport inhibitor.[6]
Quercetin is one of the most abundant dietary flavonoids,[2][3] with an average daily consumption of 25–50mg.[7]
In red onions, higher concentrations of quercetin occur in the outermost rings and in the part closest to the root, the latter being the part of the plant with the highest concentration.[8] One study found thatorganically growntomatoes had 79% more quercetin than non-organically grown fruit.[9] Quercetin is present in various kinds ofhoney from different plant sources.[10]
Naringenin is converted intoeriodictyol using flavanoid 3′-hydroxylase. Eriodictyol is then converted intodihydroquercetin with flavanone 3-hydroxylase, which is then converted into quercetin usingflavonol synthase.[11]
Thebioavailability of quercetin in humans after oral intake is very low, with one study concluding it must be less than 1%.[15] Intravenous injection of quercetin shows a rapid decay in concentration described by a two-compartment model (initialhalf-life of 8.8 minutes, terminal half-life of 2.4 hours).[15] Because it undergoes rapid and extensive metabolism, the biological effects presumed fromin vitro studies are unlikely to applyin vivo.[2][16][17][18] Quercetin supplements in theaglycone form are lessbioavailable than the quercetinglycoside often found in foods, especially red onions.[2][19] Ingestion with high-fat foods may increase bioavailability compared to ingestion with low-fat foods,[19] and carbohydrate-rich foods may increase absorption of quercetin by stimulatinggastrointestinal motility andcolonicfermentation.[2] Whereas quercetin has been shown to be a potent anti-inflammatory compound in a variety of in vitro and in vivo bioassay models, oral quercetin in human subjects has not exhibited the desired effects.[20] Because of low solubility and poor bioavailability of quercetin, derivatives have been synthesized to overcome these challenges and enhance its biological activity, leading to compounds with improved properties for possible therapeutic applications.[21]
Quercetin is rapidly metabolized (viaglucuronidation) after the ingestion of quercetin foods or supplements.[22] Five metabolites (quercetin glucuronides) have been found in human plasma after quercetin ingestion.[23][22] Taken together, the quercetin glucuronides have a half-life around 11–12 hours.[22]
Compared to otherflavonoids, quercetin is one of the most effective inducers of the phase II detoxification enzymes.[25]
In vitro studies show that quercetin is a strong inhibitor of thecytochrome P450 enzymesCYP3A4 andCYP2C19 and a moderate inhibitor ofCYP2D6.[26][27] Drugs that are metabolized by these pathways may have increased effect. Anin vivo study found that quercetin supplementation slows the metabolism ofcaffeine to a statistically significant extent in a particular genetic subpopulation, but in absolute terms the effect was almost negligible.[28]
Quercetin has been studied inbasic research and smallclinical trials.[2][30][31][32] While supplements have been promoted for the treatment of cancer and various other diseases,[2][33] there is no high-quality evidence that quercetin (via supplements or in food) is useful to treat cancer[34] or any other disease.[2][35]
The USFood and Drug Administration has issuedwarning letters to several manufacturers advertising on their product labels and websites that quercetin product(s) can be used to treat diseases.[36][37] The FDA regards such quercetin advertising and products as unapproved – with unauthorizedhealth claims concerning the anti-disease products – as defined by "sections 201(g)(1)(B) and/or 201 (g)(1)(C) of the Act [21 U.S.C. § 321(g)(1)(B) and/or 21 U.S.C. § 321(g)(1)(C)] because they are intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease",[36][37] conditions not met by the manufacturers.
Little research has been conducted into the safety of quercetin supplementation in humans, and the results are insufficient to give confidence that the practice is safe. In particular, a lack of safety information exists on the effect of quercetin supplementation for pregnant women, breastfeeding women, children, and adolescents. The hormonal effects of quercetin found in animal studies raise the suspicion of a parallel effect in humans, particularly in respect ofestrogen-dependent tumors.[38]
Quercetin supplementation can interfere with the effects of medications. The precise nature of this interaction is known for some common medicines, but for many, it is not.[38]
^abcdefghij"Flavonoids". Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis, OR. November 2015. Retrieved1 April 2018.
^Formica JV, Regelson W (1995). "Review of the biology of quercetin and related bioflavonoids".Food and Chemical Toxicology.33 (12):1061–80.doi:10.1016/0278-6915(95)00077-1.PMID8847003.
^Slimestad R, Fossen T, Vågen IM (December 2007). "Onions: a source of unique dietary flavonoids".Journal of Agricultural and Food Chemistry.55 (25):10067–80.doi:10.1021/jf0712503.PMID17997520.
^Mitchell AE, Hong YJ, Koh E, Barrett DM, Bryant DE, Denison RF, Kaffka S (Jul 2007). "Ten-year comparison of the influence of organic and conventional crop management practices on the content of flavonoids in tomatoes".Journal of Agricultural and Food Chemistry.55 (15):6154–9.doi:10.1021/jf070344+.PMID17590007.
^Petrus K, Schwartz H, Sontag G (Jun 2011). "Analysis of flavonoids in honey by HPLC coupled with coulometric electrode array detection and electrospray ionization mass spectrometry".Analytical and Bioanalytical Chemistry.400 (8):2555–63.doi:10.1007/s00216-010-4614-7.PMID21229237.S2CID24796542.
^Juergenliemk G, Boje K, Huewel S, Lohmann C, Galla HJ, Nahrstedt A (Nov 2003). "In vitro studies indicate that miquelianin (quercetin 3-O-beta-D-glucuronopyranoside) is able to reach the CNS from the small intestine".Planta Medica.69 (11):1013–7.doi:10.1055/s-2003-45148.PMID14735439.S2CID260253046.
^abGugler, R.; Leschik, M.; Dengler, H. J. (1 March 1975). "Disposition of quercetin in man after single oral and intravenous doses".European Journal of Clinical Pharmacology.9 (2):229–234.doi:10.1007/BF00614022.PMID1233267.S2CID23812714.
^Williams RJ, Spencer JP, Rice-Evans C (Apr 2004). "Flavonoids: antioxidants or signalling molecules?". (review).Free Radical Biology & Medicine.36 (7):838–49.doi:10.1016/j.freeradbiomed.2004.01.001.PMID15019969.
^Wittig, Jörg; Herderich, Markus; Graefe, Eva Ulrike; Veit, Markus (April 2001). "Identification of quercetin glucuronides in human plasma by high-performance liquid chromatography–tandem mass spectrometry".Journal of Chromatography B: Biomedical Sciences and Applications.753 (2):237–243.doi:10.1016/s0378-4347(00)00549-1.PMID11334336.
^abKing JL (2 March 2017)."Warning Letter to Cape Fear Naturals". Inspections, Compliance, Enforcement, and Criminal Investigations, US Food and Drug Administration. Retrieved29 November 2018.
^abPace R (17 April 2017)."Warning Letter to DoctorVicks.com". Inspections, Compliance, Enforcement, and Criminal Investigations, US Food and Drug Administration. Retrieved29 November 2018.
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